1. Field of the Invention
The present invention relates to an objective lens actuator that is included in an optical pickup device which projects a light beam to an optical recording medium so as to become possible recording of information or reading of information. More particularly, the present invention relates to a structure of the objective lens actuator which has a plurality of objective lenses. Further, the present invention relates to the optical pickup device which has the objective lens actuator structured as above described.
2. Description of Related Art
Optical recording media including a compact disc (hereinafter referred to as a CD) and a digital versatile disc (hereinafter referred to as a DVD) are widely spread. Furthermore in recent years, for a purpose of increasing quantity of information on the optical recording medium, studies about high density recording on the optical recording medium have been performed. For example, a high density optical recording medium such as a HD-DVD that is a high definition DVD and a Blu-Ray Disc (Registered Trademark; hereinafter referred to as a BD) are being available.
In an optical disc apparatus which performs recording or reproducing of such optical recording media, an optical pickup device is included which projects a light beam to the optical recording medium so as to become possible recording of information or reading of information. At this point it is necessary to make numerical aperture (NA) of the objective lens and a wavelength of light source which are used in the optical pickup device different depending of a kind of the optical recording medium. For example, for a CD, the NA of the objective lens of 0.50 and the wavelength of the light source of 780 nm are used, for a DVD, the NA of the objective lens of 0.65 and the wavelength of the light source of 650 nm are used, and for a BD, the NA of the objective lens of 0.85 and the wavelength of the light source of 405 nm are used.
As above described, in response to the kind of the optical recording medium, because the NA of the objective lens and the wavelength of the light source which are used become different, it is conceivable to use different optical pickup device for every different optical recording medium. However, it is more convenient that one optical pickup device can perform recording or reproducing of a plurality of kinds of optical recording media, and many of such optical pickup devices has been already developed. In such optical pickup device there are some devices which has a plurality of objective lenses such that the plurality of objective lenses are changed to utilize in response to the kind of the optical recording medium.
In the meantime the objective lens which is included in the optical pickup device is generally mounted on a lens holder that is included in the objective lens actuator so as to enable an adjustment in a focusing direction and in a tracking direction. As for this point, situation is the same in the optical pickup device which has a plurality of the objective lenses, when the plurality of objective lenses are mounted on the objective lens actuator, problems as below described are caused.
When the objective lens actuator has a plurality of the objective lenses, a tilt angle adjustment of the objective lens actuator is generally performed to enable cutting down influence of coma aberration or the like utilizing an objective lens which is used mainly in the optical pickup device. In this case, if a shift in degree of parallelization between the objective lenses (hereinafter this shift may be also referred to as relative tilt) is occurred due to shift or the like of attaching angle of the objective lens to the lens holder, the coma aberration is generated when another objective lens is used which is not utilized for the tilt angle adjustment of the objective lens actuator, therefore, quality of reading information or the like by the optical pickup device becomes worse.
Because of this, up to now, various technology which suppresses occurrence of this relative tilt, has been proposed. In JP-A-H11-312333, for example, a structure has been proposed in which three bosses are formed on a lens receiving plate of the lens holder for the optical pickup device. By this arrangement, as far as accuracy of degree of horizontally of the three bosses are maintained, degree of verticality between the objective lens and a supporting shaft can be guaranteed, and degree of parallelization between the objective lenses can be maintained each other.
Further, in JP-A-2006-019001, a structure has been proposed in which a lens attaching surface of at least any one of attaching hole among a plurality of attaching holes which are formed in the lens holder, is formed by curved surface in order to enable tilting adjusting of the objective lens. Similarly, in JP-A-2005-174485, or JP-A-H11-120602, structures have been proposed in which objective lens holding tube has curved surface or tapered surface so as to enable performing tilt angle adjustment around a principal point of the objective lens. By these arrangements, the tilt angle adjustment of the relative tilt between the objective lenses becomes possible.
However, as for the structure proposed by the JP-A-H11-312333, when mass production is thought, it is not always easy to maintain accuracy in degree of horizontally of three bosses for each of the lens attaching holes. Therefore, in the structure of JP-A-H11-312333, possibility of occurrence of the relative tilt inevitably becomes high.
Further, as for the structures proposed by JP-A-2006-019001, JP-A-2005-174485, and JP-A-H11-120602, because they have the structures in which the tilting adjustment can be performed for at least one of the objective lens between the two objective lenses, it is possible to prevent occurrence of the relative tilt. However, because the structures contain forming the curved surface or the tapered surface on the lens attaching surface or the like in order to enable tilt angle adjustment of the objective lens, dimensional control becomes necessary for the lens attaching surface or the like, as a result, an additional process for it is generated when the lens holder is manufactured. Further, because it is not always easy to slide the objective lens along a sphere or the like, in practice, a separate holder which has structure to slide smoothly along the sphere becomes necessary and the like, it causes a problem of increase in number of parts.